1. Field of the Invention
This invention relates to a portable radio transmitter system for repetitively transmitting a message, and more particularly, to a portable broadcast band transmitting system for repetitively transmitting an advertising message to be received by a vehicle-mounted broadcast band receiver.
2. Description of the Prior Art
The Federal Communication Commission permits unlicensed operation within the AM broadcast band if transmitting input power is limited to 100 milliwatts and total antenna length including transmission lines does not exceed three meters.
In the past, unlicensed, low power AM transmitting systems have utilized an endless loop cassette tape player to continuously rebroadcast an advertising message to vehicle-mounted broadcast band receivers. In 1984, such devices were priced in the range of $300-$500.00. Because of the high per unit cost, these portable transmitter systems have been positioned within a locked dwelling to minimize loss by theft. Although advertising literature claims system ranges on the order of a thousand feet, such is not the case under normal operating conditions. Most vehicular AM radio receivers possess comparatively low sensitivity and are unable to clearly receive the signals broadcast by such unlicensed transmitting systems at distances in excess of approximately seventy-five to one hundred feet. Only very costly high sensitivity automotive radio receivers can achieve significantly increased operating ranges.
In many cases, the distance between the dwelling located transmitter and the street located vehicular receiver is sufficiently great to prevent reception of the message transmitted by existing systems. This problem cannot be solved by existing systems without violating the FCC power and antenna length restrictions.
Extraneous AM band noise emanating from sources such as vehicle ignition systems, street lights, power lines, electric motors and comparable sources in combination with the low signal strength created by a dwelling-located transmitter can prevent reception by vehicle broadcast band receivers.
In order to provide compatibility between existing portable message transmitting systems and radio receivers having ditigally synthesized tuners, such existing systems have incorporated an expensive digitally synthesized transmitter frequency control system. Such complex frequency synthesizer equipment contributes to the system complexity and high cost but has been considered necessary to ensure signal reception by the growing number of narrow bandwidth digitally tuned automotive receivers.
U.S. Pat. No. 3,131,351 (Herzfeld) discloses an outdoor theater sound system using a plurality of restricted range, series connected AM transmitters. At Column 3, line 30 through Column 4, line 2, Herzfeld discloses various methods of providing a source of power to each series coupled RF oscillators. At Column 4, lines 14-37, Herzfeld teaches his intent to restrict the operating range of his transmitters to ten to twelve feet. A flexible metal ground rod 30 is provided to contact and ground each vehicle to improve the signal reception quality in view of the extraordinarily low transmitter signal strength.
U.S. Pat. No. 2,259,316 (Lowe) discloses a low power fixed location broadcast band transmitting system for transmitting a repeating message to vehicles in the vicinity of the system antenna towers. As disclosed by Lowe, the power source, audio source and transmitter are located within a single building and are coupled by a lengthy transmission line to a massive, non-portable antenna system.
U.S. Pat. No. 4,097,808 (Parke) discloses a low power AM transmitter utilizing a single strand antenna which extends over a substantial distance beside a highway.
U.S. Pat. No. 1,810,264 (Bonitz) discloses an AM receiver which is mounted within a sign. This device receives a signal broadcast from a remote transmitter and radiaties an audio output signal through a loudspeaker to pedestrians passing the sign.
U.S. Pat. No. 2,047,440 (Sloss) discloses an AM radio receiver which is tuned to receive a signal transmitted from a remote site. A sign is displayed in connection with the radio receiver loudspeaker to communicate advertisements to passing pedestrians.